1. Field of the Invention
The present invention relates generally to integrated circuit devices and more specifically to programmable logic devices which are configurable by a user.
2. Description of the Prior Art
Programmable logic devices are becoming increasingly popular in the electronics industry because of their flexibility. These devices allow a user to configure a standard part to perform a wide variety of standard logic functions. Since a single standard device can be configured many different ways, the total cost of using such a device in a system can be significantly less than the cost of custom design parts, especially when the product volume is not large. If changes or update are needed to the programmed logic function, some types of devices can be reprogrammed.
Typically, a logic function for a programmable logic device will not utilize all the input lines of the programmable logic device. These unutilized input lines are termed "don't care" inputs because they do not have an effect in the logic functions programmed in the programmable logic device.
As is known in the art, the components which define a logic device as well as the device itself consume power during operations of the device. The problem with the unutilized or "don't care" inputs is that the components corresponding to those inputs consume power during normal operation of the device, even though the inputs are not used in the programmed logic function. Consequently, the actual power needed by the programmable logic device to perform its logic function is unnecessarily increased by the components of the unutilized inputs.
Thus a need exists for a mechanism which shuts off power to the circuitry corresponding to an unused input, thus lowering the power consumed by a programmable logic device. Moreover, it is desirable that such a mechanism not adversely effect normal operation of the device.